misc/cgo/testplugin: let TestMain run deferred functions

Split TestMain into two functions so that we can defer cleanups.

Updates #30500

Change-Id: I4a5c7ddb8218a8bd056c8733c3cb9feb895e77a0
Reviewed-on: https://go-review.googlesource.com/c/164859
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Daniel Martí <mvdan@mvdan.cc>

misc/cgo/testplugin/plugin_test.go

@@ -23,7 +23,10 @@ var gcflags string = os.Getenv("GO_GCFLAGS")
 
 func TestMain(m *testing.M) {
 	log.SetFlags(log.Lshortfile)
+	os.Exit(testMain(m))
+}
 
+func testMain(m *testing.M) int {
 	// Copy testdata into GOPATH/src/testarchive, along with a go.mod file
 	// declaring the same path.
 
@@ -77,7 +80,7 @@ func TestMain(m *testing.M) {
 	goCmd(nil, "build", "-buildmode=plugin", "-o=unnamed2.so", "./unnamed2/main.go")
 	goCmd(nil, "build", "-o", "host.exe", "./host")
 
-	os.Exit(m.Run())
+	return m.Run()
 }
 
 func goCmd(t *testing.T, op string, args ...string) {